Solr's logging facility provides a wealth of information, from basic performance statistics, to what queries are being run, to any exceptions encountered by Solr. The log files should be one of the first places you look when you want to investigate any issues with your Solr deployment. There are two types of logs:
The HTTP server request style logs, which record the individual web requests made to Solr.
The Solr application logging that uses SLF4J (Simple Logging Framework for Java, a logging façade), which uses the built-in Java JDK logging facility to log the internal operations of Solr.
The HTTP server request logs record the requests that come in and are defined by the Servlet container in which Solr is deployed. For example, the default configuration for managing the server logs in Jetty is defined in jetty.xml:
<Ref id="RequestLog">
<Set name="requestLog">
<New id="RequestLogImpl" class="org.mortbay.jetty.NCSARequestLog...